GRANBY, Colo. (AP/CBS4) – A mother died and her two young daughters were injured after falling from a chairlift at a small Colorado ski area Thursday.

The 40-year-old Texas mother and the girls, ages 9 and 12, fell 20 feet from a four-person lift onto snow at Ski Granby Ranch, a 400-acre family-oriented resort about 90 miles west of Denver.

(credit: Lance Maggart/Sky-Hi Daily News)

“Ski patrol arrived, and they, in addition to EMS, transported the patients to Middle Park Medical Center in Granby,” said Schelly Olson with Granby Fire.

The 12-year-old girl was treated and released from a hospital in Granby and the 9-year-old was flown to Children’s Hospital in Aurora for treatment. Investigators have not released any names and have not said what caused the family to fall.

Police spokeswoman Schelly Olson said state regulators who oversee chairlifts are helping with the investigation.

“The lift was operating safely to get everyone else off,” Olson said.

(credit: Lance Maggart/Sky-Hi Daily News)

The Colorado Passenger Tramway Safety Board is now investigating and says the lift will reopen when it’s deemed safe for the public.

Witnesses to the accident told CBS4 there had been trouble with the lift for the past few days and the lift even stopped for 10 minutes earlier Thursday morning. Each stop of the lift caused chairs to sway and bounce.

Deaths from malfunctions or falls are rare in the United States, The Denver Post reported. According to the National Ski Areas Association, deaths from malfunctions have totaled just 12 from 1973 through October, and there were three deaths from falls between 2004 and October.

According to an October report by the NSAA, the last death on a chairlift attributed to a malfunction was in 1993. As of the 2015-2016 ski season, the annual fatality rate per 100 million miles traveled on ski lifts was 0.14.

The latest death comes during one of the busiest weeks of the season for ski resorts in Colorado.